1. Field of the Invention
The present invention relates to a plate material bending apparatus.
A plate material is gripped by an lower plate mounted on a lower frame and an upper plate mounted on a vertically movable upper frame; and an upper die holder for holding an upper bending die to bend downward an end portion of the gripped plate material and a lower die holder for holing a lower bending die to bend upward the end portion of the same gripped plate material are provided so as to be movable vertically. More specifically, the present invention relates to a plate material bending apparatus, by which the upper bending die, for instance can be retreated markedly from the bending position where a bending is performed. The bending die is moved in roughly rectilinear motion to the bending position during the bending processing; and after the end portion of the plate material has been bent at roughly a right angle, the bending die is moved along a curved locus so as to further bend the plate material (referred to as overbending).
2. Description of the Prior Art
As the prior art examples related to the plate material bending apparatus of the present invention, there have been known U.S. patent application Ser. Nos. 4,242,898; 4,356,716; 4,385,513; 4,455,857; 4,783,984; 4,843,862; 4,411,148, and Japanese Laid Open Patent Application Nos. 1-289,513; 63-60,021; 63-220,927; 63-220,928; 63-220,926; 2-274,317; and French Patent Application No. 2,187,445.
The above-mentioned prior art of plate material bending apparatuses can be divided into tow types as follows: In the first type, the upper bending die and the lower bending die are arranged on a channel-shaped holder in such a way that both the dies are opposed to each other in the vertical direction. In the second type, the upper bending die and the lower bending die are mounted on an upper holder and a lower holder, separately.
In the above-mentioned apparatuses, after the end portion of a plate material gripped between the upper plate and the lower plate has been bent in the vertical direction by moving the holder in the vertical direction, the holder is further moved slightly in the horizontal direction to achieve the overbending processing. In other words, the prior apparatus is generally provided with a driving unit for moving the holder in the vertical direction and another driving unit for moving the holder in the horizontal direction, respectively, thus raising a problem in that the construction of the apparatus is complicated and therefore costly.
A plate material bending apparatus which can move the holder in the vertical direction and further slightly in the horizontal direction by a single motor is disclosed in U.S. patent application Ser. No. 4,356,716, for instance. In the prior art apparatus of this construction, however, since the mechanism is such that the holder is moved fairly in the vertical direction and slightly in the horizontal direction by driving two different eccentric mechanisms simultaneously, it is impossible to sufficiently retreat the bending die in the vertical direction from the bending position by moving the holder.
Further, since the locus of the bending die is determined by synthesizing the motion of one eccentric mechanism for moving the holder in the vertical direction and the motion of the other eccentric mechanism for moving the holder in the horizontal direction, it is difficult to set the locus of the bending die accurately during the overbending operation.